Generally, a HEMT is known as a high-frequency element in the GHz band, which is represented by a transceiver amplifier for satellite broadcasting and the like. A typical example thereof includes one using a GaAs layer as an electron transport layer (GaAs-based HEMT). This utilizes a two-dimensional electron gas layer accumulated at a heterojunction interface formed on a GaAs substrate between AlGaAs serving as an electron supply layer and GaAs serving as an electron transport layer. Since electron mobility and electron saturation velocity is high in comparison with Si, GaAs is capable of operating a HEMT at high speed.
Here, a metamorphic-HEMT (mHEMT) is a HEMT in which the substrate material and the electron transport layer greatly differ from each other in lattice constant. In a case where the substrate and the electron transport layer greatly differ from each other in lattice constant, if the electron transport layer is formed directly on the substrate, lattice defects are generated, resulting in a problem that high electron mobility cannot be obtained. Thus, it is important to reduce this. Moreover, in the mHEMT, the larger the ΔEc at the heterointerface between the electron supply layer and the electron transport layer, the greater the maximum value of the electron concentration in the electron transport layer. This contributes to an improvement in the element properties. Further, ΔEc influences the electron distribution in the electron transport layer. The electron distribution with small ΔEc and the electron distribution with large ΔEc are illustrated in FIG. 1A and FIG. 1B, respectively. In FIG. 1A, electrons are concentrated near the heterointerface between the electron transport layer and the electron supply layer. This trend becomes prominent as the amount of donor impurity doped is increased. In contrast, in FIG. 1B, even if the amount of donor impurity doped is increased, electrons are located relatively away from the heterointerface. Since the vicinity of the heterointerface is susceptible to the interface roughness and the like, the electron mobility is expected to be higher in a case where electrons are located away from the heterointerface than a case where the electrons are concentrated near the heterointerface. In other words, the electron mobility is expected to be improved with large ΔEc.
From the above, to obtain an InxGa1-xAs electron transport layer with a high electron mobility in the mHEMT, the following two points are important: to sufficiently reduce the difference in lattice constant between the substrate and the electron transport layer; and to increase ΔEc at the heterointerface with the electron supply layer.
Meanwhile, Patent Literature 1 discloses a stacked structure in which an AlyGa1-yAsSb layer (0.3<y≦0.8) and an InxGa1-xAs layer (0.2≦y≦0.9) are sequentially formed on a GaAs substrate or a Si substrate and further an AlyGa1-yAsSb layer (0.3<y≦0.8) is formed on the layers.
Further, Non-Patent Literature 1 discloses a thin film stacked structure in which an AlGaAsSb step-graded buffer layer with discretely varied lattice constant is used as a buffer layer to reduce the difference in lattice constant between a GaAs substrate and an In0.8Ga0.2As electron transport layer. In the stacked structures in Patent Literature 1 and Non-Patent Literature 1, AlGaAsSb is used for both of the buffer layer and the electron supply layer with respect to the InGaAs electron transport layer. Since AlGaAsSb is low in electron affinity in comparison with InAlAs and InAlGaAs that are widely used in InP-based HEMTs and the like, large ΔEc can be achieved.
Additionally, Patent Literature 2 discloses using a stacked structure with an InAlAs layer adjacent to an InGaAs electron transport layer and an AlyGa1-yAszSb1-z layer (0.8≦y≦1) adjacent to the InAlAs layer as an electron supply layer to increase ΔEc.
However, in the stacked structures in Patent Literature 1 and Non-Patent Literature 1, particularly the AlGaAsSb buffer layer for reducing the difference in lattice constant between the substrate and the electron transport layer needs to have a thickness of at least 0.5 μm, or 1 μm or larger in some cases. AlGaAsSb is likely to be oxidized in comparison with InAlAs and InAlGaAs that are widely used in InP-based HEMTs and the like. Also, AlGaAsSb is difficult to perform mesa etching for element isolation, selective recess etching for exposing the surface of AlGaAsSb to form a gate electrode, and the like. These lead to a problem that the element fabrication process becomes difficult. These problems become more prominent as the thickness of the AlGaAsSb layer is increased larger and larger.
Furthermore, with the stacked structures described in these documents, the InGaAs electron transport layer does not have sufficient electron mobility. FIG. 2 illustrates a relationship between sheet electron density and normalized electron mobility in three types of stacked structures (A), (B), and (C). The sheet electron density is controlled by changing the donor impurity concentration. In the stacked structure (A), an Al0.53Ga0.47As0.2Sb0.8 layer with a thickness of 0.6 μm, an In0.8Ga0.2As layer with a thickness of 20 nm, a Sn-doped Al0.53Ga0.47As0.2Sb0.8 layer with a thickness of 35 nm, and an In0.8Ga0.2As layer with a thickness of 10 nm are sequentially stacked on a GaAs substrate. The stacked structure (B) is one described in Non-Patent Literature 1, in which the AlGaAsSb step-graded layer is used as the buffer layer. The values in the literature are used as the electron mobility values herein without any change. In the stacked structure (C), an InxAl0.3Ga0.7-xAs (x=0→0.7) graded layer with a thickness of 1 μm, an In0.7Al0.3As layer with a thickness of 350 nm, an In0.8Ga0.2As layer with a thickness of 20 nm, a Si-doped In0.7Al0.3As layer with a thickness of 16 nm, and an In0.8Ga0.2As layer with a thickness of 10 nm are sequentially stacked on a GaAs substrate. Comparing the electron mobility values, the followings can be seen: the electron mobility values are equivalent between (A) and (B) in each of which AlGaAsSb is used for both the buffer layer and the electron supply layer, and the electron mobility in the case of (C) in which InAlGaAs is used for the buffer layer and InAlAs is used for the electron supply layer is higher than those in (A) and (B). Namely, it can be said that since the effect of the AlGaAsSb buffer layer in the stacked structures (A) and (B) is insufficient in comparison with those of the InAlGaAs and InAlAs buffer layers in the stacked structure (C), the electron mobility is low although ΔEc is large.
In addition, according to the configuration described in Patent Literature 2, an InAlAs intermediate layer exists between the InGaAs layer and the AlyGa1-yAszSb1-z layer (0.8≦y≦1). This brings about a problem that abrupt heterointerface is hard to obtain.
As described above, a thin film stacked structure with large ΔEc, high electron mobility, and simplified element fabrication process has not been achieved yet.